In a communications network, switching, routing, and transponder devices (generally referred to in this application as “switches”) receive data at one of a set of input interfaces and forward the data on to one or more of a set of output interfaces. Users typically require that such switching devices operate as quickly as possible in order to maintain a high data rate. Switches are typically data link layer devices that enable multiple physical network (e.g., local area network (LAN) or wide area network (WAN)) segments to be interconnected into a single larger network. Switches forward and flood data traffic based on, for example, MAC addresses.
Because they are typically designed to accommodate high speed traffic and support high capacity data transmission, many new switches employ multi-gigabit backplane designs involving high speeds and fast edge rates. Although early backplane-based systems used wide parallel buses and fast signal clock rates, as requirements reached the one Gbit per second range it became difficult to reliably transmit data over such buses because of a variety of problems including signal skew, crosstalk, and load problems. Consequently, backplane design has shifted from parallel buses to serial interconnects. Using integrated circuit serializer-deserializer (SERDES) solutions, backplanes can transmit a serial stream that combines data and clock in the same signal.
Nevertheless, the signal integrity of these high-speed serial links is affected by a host of conditions including reflections due to impedance mismatches along the signal path, signal attenuation from backplane materials, added noise due to crosstalk and Inter Symbol Interference (ISI). Moreover, the interface between these high-speed integrated circuits becomes more important in achieving high performance, low power, and good noise immunity. Three commonly used interfaces for such high speed circuits are positive-referenced emitter-coupled logic (PECL), low-voltage differential signals (LVDS), and current mode logic (CML). When designing high-speed systems, the problem of how to connect different integrated circuits using such interfaces is often encountered.
FIG. 1 illustrates a simplified schematic diagram of a current mode logic output driver circuit coupled to a load. Current mode logic output driver 110 is in general part of an integrated circuit 100 that might be, for example, a physical layer (PHY) transceiver chip including serializer/deserializer functionality. In this example, CML output driver 110 transmits a serialized data stream through differential output nodes 120 and 130, over transmission lines (typically printed circuit board (PCB) traces), and to a load device 150 (typically an input interface for another integrated circuit). CML output driver 110 is shown AC-coupled to load device 150 using coupling capacitors C1 and C2. Although output drivers like CML output driver 110 do not necessarily need to be AC-coupled to the load they are driving, AC-coupling is often used to change the common-mode voltage level when interconnecting different physical layers. Capacitors C1 and C2 remove the DC component of the signal (common-mode voltage), while the AC component (voltage swing) is passed on to the load. For each output node 120 and 130, series resistors and inductors (R1 & L1, and R2 & L2) are added in a path back to the supply voltage VDD to enhance the performance of the AC-coupled driver and to provide a current path for the DC component.
CML output driver performance thus depends in part on the careful selection of component values for inductors L1 & L2 and resistors R1 & R2. Although this component selection can be left to the manufacturer of the printed circuit board that will included integrated circuit 100, load device 150, and electronic components L1, L2, R1, R2, C1, and C2, the designer and/or manufacturer of integrated circuit 100 may prefer to specify some or all of those components so that adequate performance is achieved. Moreover, it may be preferable, e.g., for quality control or testing, for the designer and/or manufacturer of integrated circuit 100 to select the actual components to be used. Additionally, certain performance advantages are obtained by adding the components as close to the integrated circuit as possible. Although in some cases, resistors and inductors can be included in the integrated circuit itself, this often requires large amounts of integrated circuit die space (thus increasing the overall cost of the integrated circuit) and may pose certain problems related to the design of the integrated circuit or the manufacturing process used to fabricate the integrated circuit.
Accordingly, it is desirable to have an electronic component solution that provides the desired level of performance through the use of electronic components external to the integrated circuit while still providing the designer and/or manufacturer adequate control of component selection and placement and preserving adequate levels of circuit performance.